'empirical results' indicating what? Before we look for proof we perhaps need to decide what it is we are trying to prove? I've seen claims for the KF airfoil ranging all the way from "it's better than a flat plate, easy to build and flies pretty good" (which I'm quite happy to accept)... All the way to claims that it beats conventional airfoils on all of the following: lower drag, higher lift, bigger CG range, faster top speed, slower minimum speed, stall proof, does not require dihedral, does not require reflex.. you name it.

Can you point me to just one test in all those 'thousands' where a KF airfoil has been compared to a genuine 'normal' airfoil, such as a real Clark-Y? (by 'real' Clark-Y I mean just that.. a genuine Clark-Y modelled to proper Clark-Y co-ordinates, not a slab of flat foam with a big ugly 'lump' stuck on top). You state that the KF airfoil has 'proved itself to be beneficial' as if that were a statement of fact, yet as far as I'm aware it has never been tested against 'real' conventional airfoils in anything like a scientific fashion, the only exceptions being the NASA and other wind tunnel testing that occurred in the 1970's and all of which showed the KF designs tested to perform very poorly at least in terms of L/D performance.

If we want to look for empirical evidence of the performance of airfoils on RC models than the best place to look is the wings of the winning gliders in any contest... i'll save you the trouble of trailing around hundreds of contests looking at glider wings.... you wont find a single KF anywhere

I honestly don't see anything in the 'evidence' so far, empirical or otherwise, that leads me to question the validity of conventional fluid dynamics theory to explain KF airfoil performance.

Our intention is to test multiple versions of the KF airfoil as well as a conventional airfoil of the same relative shape (an approximation of the MH32). Post 139 has the Airfoil being used for the tests. We hope to be able to visualize air flow at multiple airspeeds/angles of attack by photographing tufts while collecting data on airspeed, and power consumed.

Data points will be collected at a rate of 10/second. I plan to apply a Kaman filter to the data to smooth out the data and provide a usable set of curves.

The same airframe will be used for all wings. Aircraft weight will be kept as near the same as I can. The same wing will be used for all tests with the addition of steps and a smoothing top (for the normal airfoil). The exception will be the KFM4 wing which will have to be new construction.

The data I collect will be as accurate as I can make it, but it is not the same as can be had using a low speed wind tunnel and a smoke tunnel. I've tried to get my old Alma Matter interested in doing these tests, but the mere mention of the KF airfoil ends any meaningful discussion. Most aerodynamicists take the attitude that the KF has been studied sufficiently. Of course no studies have been done anywhere near Re 50K, much less below it. Those studies are of no use to us. Anything that would be of use to us seems to be of no interest to those who can help.

The KFM3 is a decent lifting airfoil, and it's drag is certainly not as low as the MH32, but it can make an acceptable airfoil for a training sailplane. I designed the Blu-Guppy to see if the KFM3 would work at all on a sailplane. The Blu-Guppy is a 96" EP sailplane. You can find it in the foamie forum, along with a few other EP KF sailplanes. They work. They are not competitive, nor would anyone ever expect they would be. The GAW1 airfoil and the Clark Y both have been used on many full size aircraft, and no one in the soaring community would build a competition sailplane using them either.

Even if the KF airfoil is only ever used in foam models, because it is easy to build and provides greater structural stability than a flat plate, it has it's place.

My hope is that we can actually find out a bit about how it works in our flight regimes, and where it would best be used.

Quote: " My hope is that we can actually find out a bit about how it works in our flight regimes, and where it would best be used. "
Rog , unscientifically speaking, Our years of experiences building multiples of the same model, some with KF , some not ... It is apparent the smaller size models benefit more with the simple KF I've used ... for specific intent ... Viz. The 40% underwing KF Does improve the general handling of the model lessening the stall considerably , as has been described before , "like a cushion of air"..
The KF @ 50% on the top of the wing , does improve the glide of a Flat Plate airfoil ...
The proof is in the flying eh !!!!
If we take the KF "in context" with the flat plate airfoil , the benefit is immediately apparent ...

Without some "commercial potential " , the search for the truth becomes rather Limited... The glider guys used to complain about this in the old days when they were testing their Addidas & Rebock "airfoils" ...

Yeah, my main concern is the advantage offered by the KF over a flat plate.The mega-cool thing about a simple folded KFm2 is that you don't have to spend hours of labor in measuring, cutting, gluing, clamping, yada, yada, and more yada -- just to end up with a wing. I am a lot more interested in flying than I am in building, and the KF provides a simple, easy, quick, inexpensive, and above all, low labor means of building a wing of adequate stiffness, performance, and reliability.

I have no idea at all if any variation of the KF will ever out-perform some conventional airfoil. This comparison does not apply to me. The effectiveness of the KF is that it requires many fewer hours of design, calculation, drawing, scaling, wielding of a shape knife, and everything else involved in making a Clark Y or whatever. In return for that reduction in labor I get a wing that has proven itself to be strong, durable, forgiving, easy to build, and easy to repair. So what if it doesn't have the absolute minimum drag coefficient per lift? I can live with that.

Of course I'd like to know more about the science of why the KF does what it does. (Like everybody else, I simply don't know what is happening when the KF cuts through the air.) To that end I am willing to entertain wild speculation and bizarre creative thinking, since the typical airfoil models break down when applied to the KF. A refined science behind the KF will tell us what to change on our aircraft to achieve higher levels of performance. That is what I am really after; the science is just a means to an end.

If all we are trying to explain is 'why does a KF often perform better than a flat plate'?.. than I don't think we need to create any new science at all.

The top step KF airfoil basically mirrors a conventional airfoil with the rear cut away.. The cut-away might degrade performance a little compared to a conventional airfoil but the fact that the front half of the airfoil is closer to a normal cambered airfoil shape means that it will still perform better than a pure flat plate. Getting into the detail, the more rounded nose and curved upper surface profile near the LE will help prevent laminar separation bubbles forming and/or total gross separation occurring at the LE. This is all conventional airfoil theory, you don't need 'new science' to explain it.
If the bottom step (flat topped) KF were claimed to significantly out perform a flat plate then that would be harder to explain with conventional theory but most seem to have abandoned bottom step KF airfoils in favour of the top step because, as conventional theory would predict, top step design performs better.

Conventional airfoil modelling tools like X-foil may have a hard time modelling KF airfoils. This is because X-Foil is designed to model streamline shapes that don't have large areas of separation, not stepped shapes that inherently do suffer separation. However just because some software tools struggle to handle stepped airfoils doesn't mean there is not already a perfectly valid scientific explanation for what's going on.

So are we all in agreement that KF's can perform better than flat plates but cant out perform (in pure efficiency terms) properly designed high performance 'conventional' airfoils?.. or are there those out there that still think they knock spots of all other airfoils in every way possible

One more for the KFm1... 8 foot flying wing.

Here a tiny little motor pushes this aircraft up and away. Later in the year, he was able to carry 3 pounds of candy aloft at an indoor event and drop the sweets on a field where kids could pick them up. He had no problem lifting the weight up to the height he wanted.